Four quark states and glueballs

In this talk a possible evidence of Q/sup 2/Q-bar/sup 2/ mesons in γγ → rho/sup o/ rho/sup o/ and rho/sup +/ rho/sup -/ reactions is presented. Also the effect of charm quark mass in the process J/psi(psi) → γ/sup +/ glueball is discussed. This effect is used to explain the suppressed radiative decay mode of psi/sup '/ (3684). In hadron physics the spectroscopy of the s-wave Q/sup 2/Q-bar/sup 2/ mesons has been studied in both the MIT bag model and the potential model. The salient features of these states are the following: (1) The wavefunctions of the Q/sup 2/Q-bar/sup 2/ states consists of two parts. In one part, the QQ-bar pairs are in color singlet representation and in another part the QQ-bar pairs are in the color octet representation. (2) Their decays obey the OXI rule. Most of the Q/sup 2/Q-bar/sup 2/ states can ''fall apart'' into two constituent color-singlet QQ-bar mesons, making them too broad to be observed. However, there are some exceptions which decay to two vector mesons dominantly and whose masses just beyond the threshold of corresponding two vector mesons, hence their widths are not too broad to be detected as ordinary ''mass bump''